1. Technical Field of the Invention
This invention relates to methods and apparatus for the fabrication of micro-electro-mechanical systems (MEMS), micro-opto-mechanical systems (MOEMS), surface micro machined systems, and similar wafer-mounted microstructures; and in particular to methods and apparatus for applying supercritical fluid drying techniques in the fabrication of microstructures.
2. Background Art
One method of manufacturing micro-electro-mechanical systems (MEMS) based devices is Sacrificial Surface Micromachining (SSM) or surface micromachining. FIG. 1 is a prior art illustration of a simple xe2x80x9canchoredxe2x80x9d SSM silicon based production process. In FIG. 1a, a substrate, such as Silicon, is deposited with a sacrificial material, such as grown Silicon Dioxide or SiO2. In FIG. 1b, the sacrificial material is etched to open a hole for the anchor of the structure. In FIG. 1c, a structural material such as polysilicon is deposited on the sacrificial material. In FIG. 1d, the sacrificial material is etched away to release the structural layer, creating the microstructure. These steps can be repeated to form more complex multilevel structures. Although SiO2 is a common material for a sacrificial layer, other materials like photoresists may be used in other applications.
After removal of the sacrificial material or sacrificial layer by etching or other methods the wafer has to be rinsed to remove any residual trace of the etch liquid. Rinsing usually is done with deionized water, which causes the problem of stiction upon drying.
Stiction or adhesion occurs when a xe2x80x9creleasedxe2x80x9d structure adheres to another surface. FIG. 2 gives a visual representation of stiction and how it is generated. FIG. 2a, shows a properly released cantilevered polysilicon beam with rinsing liquid still trapped under it. FIG. 2b, shows how the capillary force generated upon drying of the rinsing liquid pulls the beam towards the silicon substrate. FIG. 2c, shows how the beam sticks to the substrate, rendering the device flawed.
The capillary force, responsible for the deformation of a beam upon drying as illustrated in FIG. 2, is represented by the following equation:       F    m    =                    γ        ⁢                  xe2x80x83                ⁢        A            h        ⁢          (                        cos          ⁢                      xe2x80x83                    ⁢                      θ            1                          +                  cos          ⁢                      xe2x80x83                    ⁢                      θ            2                              )      
where xcex3 is the surface tension of the rinsing liquid, A is the surface area that the beam shares with the substrate, h is the height of the gap between the surface of the substrate and the beam, and xcex81 and xcex82 are the contact angles of the rinsing liquid with the substrate and the beam, respectively.
There are two methods of controlling the capillary force, (i) manipulating the contact angle of the rinsing liquid by modifying the surface tension of the rinsing liquid, or (ii) reducing or eliminating the surface tension xcex3. The first method can only minimize the capillary force since the conditions of the surfaces in contact with the rinsing liquid, determining the contact angle, can vary. In addition, the condition of the rinsing liquid can vary during its use and may lead to unpredictable stiction and loss of yield.
Working to find improvements to the controlled release of microstructures without subsequent sticking of these structures to the substrate, researchers at the University of California at Berkeley have developed a process for drying silicon wafers in a supercritical fluid environment. In it""s supercritical state, xcex3, the surface tension is zero, and therefore capillary forces cannot be built up as can be easily seen in the equation. If it is now possible to keep the environment surrounding the structure in a state with xcex3=0 during the whole drying process, stiction never occurs. The supercritical fluid of choice was CO2, carbon dioxide, due to its low critical point, determined by a critical temperature TC of 31.1 degrees centigrade and a critical pressure pC of 1073 pounds per square inch over atmosphere.
Before CO2 can be applied for drying, an intermediate process step has to be introduced based on the fact, that water, the rinsing liquid applied after the sacrificial etch step, is not miscible with CO2. After rinsing, when the wafer still is wet with water, the water has to be replaced by a material that is miscible with CO2. This material can be methanol or any other material that is to 100% miscible with CO2. Furthermore, the wafer has to be kept submerged in methanol till it is safely deposited in the process chamber.
Using this laboratory method, a silicon wafer containing a pattern of microelectronic structure, having been fabricated in the conventional manner, but with the added step of replacing the rinsing liquid water by methanol, is introduced into a pressure vessel, with a horizontal orientation, submerged in methanol. To accomplish this, the pressure vessel is first filled with methanol. Then the operator quickly transfers the wafer into the vessel while deftly attempting to maintain a liquid layer of methanol on the wafer surface during this transport. The pressure vessel is then sealed, and a through-flow of liquid carbon dioxide is introduced for about 15 minutes. The methanol is rapidly absorbed into the liquid carbon dioxide and carried out of the pressure vessel. When the vessel has been entirely purged of methanol and is completely filled with pure liquid carbon dioxide, heat is applied uniformly for several minutes, causing the carbon dioxide to transition to its supercritical phase.
It is at this point that the benefit of the process is realized, as no liquid/vapor interface occurs during this transition. The CO2 is then slowly vented to atmosphere. With the temperature kept higher than the critical temperature during venting, CO2 does not experience a phase transition and remains in a state with the surface tension equal to zero.
The prior art pressure vessel used in the laboratory setup for demonstrating this process is shown in FIG. 3. As is readily apparent from the figure, a vessel that can be opened in cross section and when closed is subjected to elevated temperature and pressure to this extent must be of substantial construction, with a locking mechanism adequate to safely sustain the total pressure applied. In the laboratory set up, a circumferential pattern of 8 bolts is used to secure the top to the base of the vessel, to contain the high pressure. Heat is applied to the vessel by external heaters, and ports in the vessel admit and remove the materials of the process.
There are several obvious problems with the laboratory set up that must be addressed in order to make this process sufficiently cost-effective and efficient for use in a production environment. The device is not suitable for integration into a production line with automated means for inserting and removing wafers; there is no safe transfer mechanism to ensure that a liquid layer is maintained on the wafer during the transport or transfer process; the closing mechanism of the pressure vessel is manual and too slow; and the serially administered steps of the process are manually accomplished and too slow for production requirements. The device is also lacking the safeguards required by industrial standards and regulations for production requirements.
During manufacturing, once the sacrificial layer is removed, if for any reason the wafer becomes dry, it can result in stiction or adhesion of devices onto the wafer substrate. Hence the transport of wafer from one manufacturing step to another manufacturing step without causing stiction is always a problem, and usually has been dependent on operator efficiency resulting in low device yields.
The invention, in its simplest form, is an apparatus and method for implementing and improving on the prior art methods for the drying of micro-electro-mechanical structures on silicon wafers or other substrate material or drying of wafers in general.
It is therefore an object of the invention to provide a practical and safe production mechanism for the CO2 supercritical phase drying of wafers, and of microstructures on substrates.
It is a further object to provide for emplacement of the wafers or substrates into the pressure vessel submerged in a first process fluid or rinsing agent such as methanol, and to then directly displace the methanol with a second process fluid also in a liquid state, such as liquid carbon dioxide, this being connected within the pressure vessel and without disturbing the microstructures.
It is a yet further object to then elevate the second process fluid to supercritical state so as to cause the drying of the wafers with the benefits of the supercritical process, then reducing the pressure and temperature for recovery of the wafers.
Still other objects and advantages of the present invention will become readily apparent to those skilled in this art from the following detailed description, wherein we have shown and described only a preferred embodiment of the invention, simply by way of illustration of the best mode contemplated by me on carrying out our invention.